166 MORPHOLOGY AND EVOLUTION (1) Attolens, attrahens, retrahens and tragicus, all separate. (2) Attolens and attrahens fused, retrahens and tragicus separate, (3) Attrahens and retrahens fused, attolens and tragicus separate. The tragicus is frequently absent. The masseter (fig. 29) is composed of two parts in all the Apes ; but the Chimpanzee appears to be the only species in which there is a strong aponeurotic sheet between the two parts and giving origin to both. The superficial part runs from the lower border of the malar bone and anterior two-thirds of the zygoma; the fibres run downwards and backwards to the lower border, angle and posterior border of the mandibular ramus. The deep part, from the entire length of the deep surface of the zygoma, is attached to the anterior two-thirds of the outer surface of the ascending ramus of the mandible- and the coronoid process. Numerous large vessels ramify between the masseter and parotid gland and supply both. The temporal muscle (fig. 27B) is always powerful. As age advances its origin spreads upwards. In the adult Gorilla and Orang the large sagittal crest gives it an extended origin. It springs from the temporal fossa and the temporal fascia; and it is inserted into the anterior border, point and posterior border of the coronoid process. The muscular mass contains tendinous fibres, and its posterior part is very powerful. The deep temporal arteries ramify within it. The pterygoid muscles (fig. 30, A) are disposed as in Man, and their relations to the vessels and nerves in the pterygoid space are similar.